1. Field of the Invention
The present invention relates generally to speech processing, and more particularly to a sub-band hybrid codec for achieving high quality synthetic speech by combining waveform coding in the baseband with parametric coding in the high band.
2. Description of the Prior Art
The present invention combines techniques common to waveform approximating coding and parametric coding to efficiently perform speech analysis and synthesis as well as coding. These two coding paradigms are combined in a codec module to constitute what is referred to hereinafter as Sub-band Hybrid Vocoding or simply Hybrid coding.
The present invention provides a system and method for processing audio and speech signals. The system encodes speech signals using waveform coding in the baseband in combination with parametric coding in the high band. In one embodiment, the waveform coding is implemented by separating the input signal into at least two sub-band signals and encoding one of the at least two sub-band signals using a first encoding algorithm to produce an encoded output signal; and encoding another of said at least two sub-band signals using a second encoding algorithm to produce another encoded output signal, where the first encoding algorithm is different from the second encoding algorithm. In accordance with the present disclosure, the present invention provides an encoder that codes N user defined sub-band signals in the baseband with one of a plurality of waveform coding algorithms, and encodes N user defined sub-band signals with one of a plurality of parametric coding algorithms. That is, the selected waveform/parametric encoding algorithm may be different in each sub-band.
In another embodiment, the waveform coding is implemented by a relaxed code excited linear predictor (RCELP) coder, and the high band encoding is implemented with a Harmonic coder. In this embodiment, the encoding method generally comprises the steps of: separating an input speech/audio signal into two signal paths. In the first signal path, the input signal is low pass filtered and decimated to derive a baseband signal. The second signal path is the full band input signal. In one embodiment, at an analysis stage, the fullband input signal is encoded using a Harmonic coding model and the baseband signal path is encoded using an RCELP coding model. The RCELP encoded signal is then combined with the harmonic coded signal to form a hybrid encoded signal.
According to one aspect of the present invention, during synthesis the decoded signal is modeled as a reconstructed sub-band signal driven by the encoded baseband RCELP signal and fullband Harmonic signal. The baseband RCELP signal is reconstructed and low pass filtered and resampled up to the fullband sampling frequency while utilizing a sub-band filter whose cutoff frequency is lower than the analyzers original low pass filter. The fullband Harmonic signal is synthesized while maintaining waveform phase alignment with the baseband RCBLP signal. The fullband Harmonic signal is then filtered using a high pass filter complement of the sub-band filter used on the decoded RCELP baseband signal. The sub-band RCELP and Harmonic signals are then added together to reconstruct the decoded signal. The hybrid codec of the present invention may advantageously be used with coding models other than Waveform and Harmonic models.
The present disclosure also contemplates the simultaneous use of multiple waveform encoding models in the baseband, where each model is used in a prescribed sub-band of the baseband. Preferable, but not exclusive waveform encoding models include at least a pulse code modulation (PCM) encoder, an adaptive differential PCM encoder, a code excited linear prediction (CELP) encoder, a relaxed CELP encoder and a transform coding encoder.
The present disclosure also contemplates the simultaneous use of multiple parametric encoding models in the high band, where each model is used in a prescribed sub-band of the highband. Preferable, but not exclusive parametric encoding models include at least a sinusoidal transform encoder, harmonic encoder, multi band excitation vocoder (MBE) encoder, mixed excitation linear prediction (MELP) encoder and waveform interpolation encoder.
A further advantage of the present invention is that the hybrid codec need not be limited to LPF sub-band RCELP and Fullband Harmonic signal paths on the encoder. The codec can also use more closely overlaping sub-band filters on the encoder. A still further advantage of the hybrid codec is that parameters need not be shared between coding models.